Elastomer-reinforced polymers of monovinylidene aromatic compounds such as styrene, alpha-methylstyrene and ring-substituted styrene are used extensively in wide ranging commercial applications. For example, elastomer-reinforced styrene polymers having discrete elastomer particles such as cross-linked rubber dispersed throughout the styrene polymer matrix can be useful for a wide range of applications including food packaging, office supplies, automobile component parts, houseware and consumer goods, building insulation and cosmetics packaging. Such elastomer-reinforced polymers are commonly referred to as high impact polystyrene (HIPS).
Methods for the production of polymers, such as HIPS, can employ polymerization using a continuous flow process. Continuous flow processes involve apparatuses including a plurality of successively arranged reactors wherein the degree of polymerization increases from one reactor to the next. The reactor types used in the production of HIPS can include continuously stirred tank reactors (CSTR) and/or plug flow reactors (PFR). Factors such as the arrangement of the reaction vessels and the reaction conditions influence the characteristics of the HIPS produced. The extent of polymerization within each reactor, resulting in differing mechanical and/or optical properties, as well as the amount elastomer content may determine the grade of HIPS produced.
The physical characteristics and mechanical properties of HIPS are dependent upon many factors such as the particle size of the cross-linked rubber particles. An important quality of HIPS material is the ability of such material to resist environmental stress cracking. This ability must be combined with the characteristic of high impact strength in order to be useful in articles such as food containers. In addition, other important properties for such articles include flexural strength and tensile strength.
For HIPS or any other thermoplastic polymer used in the preparation of food containers, the property of stress crack resistance, or environmental stress crack resistance (ESCR), is particularly important. The food content of such polymer containers might not normally degrade HIPS or any other type of polymeric material of which the container is made, but when a thermoplastic polymer is thermoformed from extruded sheet material, residual stresses are locked into the molded article. These stresses open the polymer up to attack by substances of which the polymer would normally be impervious to. Such articles made from styrene polymers modified with rubber to increase impact strength are prone to stress cracking when they come into contact with common agents found in organic food products such as fats and oils. Likewise, such products are also subject to stress cracking when coming into contact with organic blowing agents such as halohydrocarbons, containing fluorine and chlorine. These polymers generally are found in household items such as refrigerator liners, which may crack when the cavities in the refrigerators are filled with polyurethane foam as a result of the blowing agent utilized in the foam.
In the past, efforts to prevent or mitigate environmental stress cracking consisted of complex procedures usually involving multiple layer polymer construction wherein an intermediate protective layer of polymer is placed between a polystyrene layer and a blowing agent or a polystyrene layer and a fatty food material. One such layer of material utilized to insulate the styrene from these agents is a terpolymer material known as ABS, or acrylonitrile-butadiene-styrene. Other attempts to improve the stress crack resistance of high impact monovinylaromatic polymers include increasing the amount of rubber mixed in the polymer. The higher rubber content, however, can decrease the tensile and flexural strengths, and will typically increase the cost.
The type of continuous flow processes used, as well as the amount of elastomer utilized, greatly influences the cost of HIPS production. Thus it would be desirable to develop an apparatus and methodology for the production of HIPS having reduced elastomer content with enhanced mechanical properties, such as impact strength, ductility, and ESCR. It would also be desirable to develop an apparatus and methodology for the production of HIPS having improved environmental stress crack resistance.